The army would be highly heterogeneous, and almost certainly not built to purpose. Consider the fleet of Tesla vehicles to be one source of ready-to-hack kinetic impactor drones and incendiary devices. But also... militaries around the world are developing murderbot tech. An Amazon delivery drone could drop a brick from 20 feet up.

The self driving vehicles could target food delivery -- not your ubereats, but farm trucks and roads. Destruction of logistic infrastructure results in mass starvation. Individual targets for a complete eradication would take more effort, but a significant cull of the global population seems quite feasible with today's tech.

But folks are also doing virus research with AI. If such a lab gets hacked to make a super-virus, and its containment gets breached by a kinetic impactor, it could be cataclysmic.